Belt for papermaking and process for producing papermaking belt

ABSTRACT

In a papermaking belt, prevented from cracking and inhibited from growth of a crack, comprising a reinforcing substrate ( 6 ) embedded in a thermosetting polyurethane layer ( 7 ) so that the said reinforcing substrate ( 6 ) and the said thermosetting polyurethane layer ( 7 ) are integrated with each other and the outer peripheral surface and the inner peripheral surface of the belt are formed by polyurethane layers, the polyurethane layer forming the outer peripheral surface is made of a composition containing a urethane prepolymer having isocyanate groups on ends and a hardener containing dimethylthiotoluenediamine.

TECHNICAL FIELD

[0001] The present invention relates to a papermaking belt. Morespecifically, it relates to an improvement of polyurethane in apapermaking belt formed by integrating a reinforcing substrate and athermosetting polyurethane layer with each other. It also relates to animprovement in a method of manufacturing a papermaking belt formed byintegrating a reinforcing substrate and a thermosetting polyurethanelayer with each other. In particular, the present invention is used fora shoe pressing belt, a calender belt and a sheet transfer belt used forthe paper industry.

BACKGROUND ART

[0002] In recent years, the so-called shoe pressing dehydrating a wetweb by pressing one surface of the wet web placed on a felt membertraveling at a high speed with a press roll while pressurizing the othersurface with a pressure shoe through an endless belt is widely used in apress part of a papermaking step, in order to improve the dehydrationeffect for the wet web. In the shoe pressing, a belt endlessly formed byintegrating a reinforcing substrate and a thermosetting polyurethanelayer is employed in general. Also in a calender step of smoothing andglossing the surface of paper, employment of the aforementioned elasticbelt is recently studied. In addition, employment of the aforementionedelastic belt is studied also as to a sheet transfer belt for preventinga web break and stably transporting a wet web particularly whenperforming papermaking at a high speed. Japanese Utility ModelLaying-Open No. 59-54598, Japanese Patent No. 2889341, Japanese PatentNo. 3045975 or the like discloses a typical structure of such apapermaking belt prepared by covering both surfaces of a fabric basewith an elastic material. Japanese Patent No. 2542250 or the likediscloses another typical structure prepared by embedding reinforcingyarns in an elastic material.

[0003] As an elastic material for the papermaking belt, thermosettingpolyurethane prepared by mixing a urethane prepolymer and a hardenerwith each other and hardening the mixture is generally used as disclosedin Japanese Patent No. 2889341, Japanese Patent Laying-Open No.6-287885, Japanese Patent No. 3045975, Japanese Patent No. 3053374,Japanese Patent Laying-Open No. 11-247086 or the like, and4,4′-methylene-bis-(2-chloroaniline) (hereinafter referred to as “MOCA”)is employed as the hardener in thermosetting polyurethane used for thepapermaking belt.

[0004] Generally in the shoe pressing, severe bending and pressing arerepeated on the belt between the press roll and the pressure shoe, andhence a polyurethane layer forming the belt is disadvantageouslycracked. This cracking is mainly caused on the outer peripheral surfaceof the belt coming into contact with the felt member or paper. Whilegrooves are generally formed on the outer peripheral surface of a beltfor a dehydrating press used in the press part for improving thedehydrating efficiency, the said cracking is readily caused on thebottom edges and the top edges of these grooves in particular. A crackonce caused tends to grow into a larger crack as the belt is used. Inthis case, lubricating oil stored between the inner peripheral surfaceof the belt and the pressure shoe externally leaks to exert badinfluence on the paper or causes delamination of the belt. Thus,occurrence and growth of the crack cause reduction of the life of thebelt. Therefore, suppression of occurrence and growth of cracks isstrongly demanded in relation to a papermaking belt employed for shoepressing or the like. Further, delamination may result from weakadhesion between the fabric base and the polyurethane layer, andprevention of such delamination of the fabric base and the polyurethanelayer is strongly demanded.

DISCLOSURE OF THE INVENTION

[0005] The present invention solves the aforementioned problems, and anobject thereof is to provide a papermaking belt, formed by integrating areinforcing substrate and a thermosetting polyurethane layer with eachother, capable of preventing cracking. Another object of the presentinvention is to provide a papermaking belt, formed by integrating areinforcing substrate and a thermosetting polyurethane layer with eachother, capable of suppressing growth of a crack even if the crack iscaused on the papermaking belt. Still another object of the presentinvention is to provide a papermaking belt, formed by integrating areinforcing substrate and a thermosetting polyurethane layer with eachother, capable of suppressing delamination between the reinforcingsubstrate and the polyurethane layer. A further object of the presentinvention is to provide a method of manufacturing the aforementionedpapermaking belt.

[0006] The papermaking belt according to the present invention is apapermaking belt comprising a reinforcing substrate embedded in athermosetting polyurethane layer and having an outer peripheral surfaceand an inner peripheral surface formed by the said polyurethane layer,while a polyurethane layer forming the outer peripheral surface is madeof a composition containing a urethane prepolymer having isocyanategroups on ends and a hardener containing dimethylthiotoluenediamine.

[0007] In the papermaking belt according to the present invention, thesaid urethane prepolymer and the said hardener are mixed with each otherin a ratio setting the value of the equivalent ratio (H/NCO) betweenactive hydrogen groups (H) of the said hardener containingdimethylthiotoluenediamine and the isocyanate groups (NCO) of the saidurethane prepolymer to 1<H/NCO<1.15.

[0008] The papermaking belt according to the present invention is apapermaking belt comprising a reinforcing substrate embedded in athermosetting polyurethane layer and having an outer peripheral surfaceand an inner peripheral surface formed by the said polyurethane layer,while a polyurethane layer forming the outer peripheral surface is madeof a composition containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on ends, andthe said urethane prepolymer and the said hardener are mixed with eachother in a ratio setting the value of the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the said hardener and theisocyanate groups (NCO) of the said urethane prepolymer to 1<H/NCO<1.15.In the present invention, the equivalent ratio is a stoichiometricequivalent ratio.

[0009] The papermaking belt according to the present invention is apapermaking belt comprising a reinforcing substrate embedded in athermosetting polyurethane layer with the said polyurethane layerincluding an inner polyurethane layer and an outer polyurethane layeradhering to the outer peripheral surface of this inner polyurethanelayer, while each of the said inner polyurethane layer and the saidouter polyurethane layer is made of a composition containing a urethaneprepolymer having isocyanate groups on ends and a hardener having activehydrogen groups on ends, the composition forming the said innerpolyurethane layer is prepared by mixing the urethane prepolymer and thehardener with each other in a ratio setting the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 0.85≦H/NCO<1, andthe composition forming the said outer polyurethane layer is prepared bymixing the urethane prepolymer and the hardener with each other in aratio setting the value of the said equivalent ratio (H/NCO) to1<H/NCO<1.15.

[0010] In the papermaking belt according to the present invention, theadhesive surface between the said inner polyurethane layer and the saidouter polyurethane layer is present in the said reinforcing substrate,the urethane prepolymer forming the said inner polyurethane layercontains a urethane prepolymer obtained by reacting polyol anddiphenylmethane diisocyanate (MDI) with each other, and the urethaneprepolymer forming the said outer polyurethane layer contains a urethaneprepolymer obtained by reacting polyol and tolylene diisocyanate (TDI)with each other.

[0011] In the papermaking belt according to the present invention, atleast 50 wt. % of the hardener forming the said inner polyurethane layeris preferably polyol.

[0012] In the papermaking belt according to the present invention, thesaid reinforcing substrate preferably contains multi-woven fabric.

[0013] In the papermaking belt according to the present invention, thesaid outer polyurethane layer adheres to the outer peripheral surface ofthe said inner polyurethane layer, forms the outer peripheral surface ofthe papermaking belt, and is made of a composition containing a urethaneprepolymer having isocyanate groups on ends and a hardener containingdimethylthiotoluenediamine.

[0014] In the papermaking belt according to the present invention, thesaid thermosetting polyurethane layer includes an inner polyurethanelayer, an outer polyurethane layer adhering to the outer peripheralsurface of this inner polyurethane layer and a polyurethane layerlocated on the outer side of this outer polyurethane layer for formingthe outer peripheral surface of the papermaking belt, and the saidpolyurethane layer forming the outer peripheral surface is made of acomposition containing a urethane prepolymer having isocyanate groups onends and a hardener containing dimethylthiotoluenediamine.

[0015] In the papermaking belt according to the present invention, thecomposition of the said polyurethane layer forming the outer peripheralsurface is preferably prepared by mixing the said urethane prepolymerand the said hardener with each other in a ratio setting the value ofthe equivalent ratio (H/NCO) between the active hydrogen groups (H) ofthe said hardener and the isocyanate groups (NCO) of the said urethaneprepolymer to 1<H/NCO<1.15.

[0016] In the papermaking belt according to the present invention, thesaid polyurethane layer is preferably hardened at a temperature of 120°C. to 140° C.

[0017] In another papermaking belt according to the present invention,grooves are formed on the outer peripheral surface thereof.

[0018] The method of manufacturing a papermaking belt according to thepresent invention is a method of manufacturing a papermaking beltincluding an inner polyurethane layer and an outer polyurethane layeradhering to the outer peripheral surface of this inner polyurethanelayer by embedding a reinforcing substrate in a thermosettingpolyurethane layer thereby integrating the said reinforcing substrateand the said thermosetting polyurethane layer with each other, includinga first step of hardening a liquid mixture, containing a urethaneprepolymer having isocyanate groups on ends and a hardener having activehydrogen groups on ends, prepared by mixing the urethane prepolymer andthe hardener with each other in a ratio setting the value of theequivalent ratio (H/NCO) between the active hydrogen groups (H) of thehardener and the isocyanate groups (NCO) of the urethane prepolymer to0.85≦H/NCO<1 at a temperature of 70° C. to 100° C. for forming the saidinner polyurethane layer, a second step of applying a liquid mixture,containing a urethane prepolymer having isocyanate groups on ends and ahardener having active hydrogen groups on ends, prepared by mixing theurethane prepolymer and the hardener with each other in a ratio settingthe value of the equivalent ratio (H/NCO) between the active hydrogengroups (H) of the hardener and the isocyanate groups (NCO) of theurethane prepolymer to 1<H/NCO<1.15 onto the outer peripheral surface ofthe said inner polyurethane layer, and a third step of heating the wholeto a temperature of 120° C. to 140° C. for hardening the liquid mixtureapplied onto the outer peripheral surface of the inner polyurethanelayer and forming the outer polyurethane layer while bonding andintegrating the inner polyurethane layer and the outer polyurethanelayer to and with each other.

[0019] In another method of manufacturing a papermaking belt accordingto the present invention, the said reinforcing substrate is impregnatedwith the said inner polyurethane layer from one surface side of the saidreinforcing substrate to an intermediate portion of the thickness of thesaid reinforcing substrate, and the said reinforcing substrate isimpregnated with the said outer polyurethane layer from the othersurface side of the said reinforcing substrate to the position where thesaid reinforcing substrate is impregnated with the said innerpolyurethane layer.

[0020] In the method of manufacturing a papermaking belt according tothe present invention, the said reinforcing substrate preferablycontains multi-woven fabric.

[0021] Still another method of manufacturing a papermaking beltaccording to the present invention includes a step of winding the saidreinforcing substrate on the outer peripheral surface of the said innerpolyurethane layer before or after hardening the said inner polyurethanelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is an explanatory diagram showing a shoe pressingapparatus.

[0023]FIG. 2 is a local sectional view showing an exemplary papermakingbelt according to the present invention.

[0024]FIG. 3 is a local sectional view showing another exemplarypapermaking belt according to the present invention.

[0025]FIG. 4 is a local sectional view showing still another exemplarypapermaking belt according to the present invention.

[0026]FIG. 5 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0027]FIG. 6 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0028]FIG. 7 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0029]FIG. 8 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0030]FIG. 9 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0031]FIG. 10 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0032]FIG. 11 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0033]FIG. 12 is a local sectional view showing a further exemplarypapermaking belt according to the present invention.

[0034]FIG. 13 is a local sectional view showing the structure of afabric base used for Example of the present invention.

[0035]FIG. 14 is a local sectional view showing the structure of asample used for a comparative experiment of the present invention.

[0036]FIG. 15 a diagram illustrating a test apparatus for crackresistance.

[0037]FIG. 16 illustrates results of a crack resistance test.

BEST MODES FOR CARRYING OUT THE INVENTION

[0038]FIG. 1 shows an exemplary shoe pressing apparatus used for a presspart of a papermaking step. Referring to FIG. 1, a flexible cylindricaldehydrating press belt 2 is provided under a press roll 1. A felt member3 and a wet web 4 are passed between the belt 2 and the press roll 1.The outer peripheral surface of the belt 2 is in direct contact with thefelt member 3. A pressure shoe 5 is pressed against the inner peripheralsurface of the belt 2 toward the press roll. Lubricating oil is suppliedbetween the pressure shoe 5 and the belt 2 for smoothly running the belt2. The belt 2 travels while sliding on the pressure shoe 5 due tofriction with the felt member 3. The pressure shoe 5 has a concavesurface corresponding to the surface of the press roll 1. Apressurizing/dehydrating part P having a large width is formed betweenthe press roll 1 and the pressure shoe 5. This pressurizing/dehydratingpart dehydrates the wet web 4.

[0039]FIG. 2 is a local sectional view showing an exemplary belt 2. Thisbelt is an endless belt formed by integrating a fabric base 6 defining areinforcing substrate and a thermosetting polyurethane layer 7 with eachother. The fabric base 6 is made of organic fiber such as polyamide orpolyester. The fabric base 6 is impregnated and covered with thepolyurethane layer 7 consisting of a monolayer. The outer peripheralsurface and the inner peripheral surface of the belt are formed by thepolyurethane layer 7.

[0040] In order to prepare the belt shown in FIG. 2, a relatively openweave endless fabric base 6 capable of passing liquid polyurethane isemployed. A plain-woven fabric base having 10 to 100 meshes can be usedas the open weave fabric base. The term “mesh” stands for the number ofyarns per inch. A papermaking belt formed by integrating the fabric base6 and the polyurethane layer 7 with each other so that the fabric base 6is embedded in the polyurethane layer 7 can be manufactured by arrangingthe said fabric base 6 on a mandrel while defining a clearance betweenthe same and the mandrel and casting the polyurethane layer 7 fromabove.

[0041]FIG. 3 shows an example employing reinforcing yarns 8 and 9 as areinforcing base substrate in place of the fabric base 6 of the beltshown in FIG. 2. In the belt shown in FIG. 3, the reinforcing yarns 8and 9 are embedded in a thermosetting polyurethane layer 7 consisting ofa monolayer. The reinforcing substrate is formed by the yarns 8 in thebelt traveling direction (hereinafter referred to as an “MD”) and theyarns 9 in a direction (hereinafter referred to as a “CMD”)perpendicular thereto. A number of yarns 8 in the MD and a number ofyarns 9 in the CMD are arranged substantially at regular intervals. Forexample, polyamide, aromatic polyamide, polyester or the like can beused as the material for the yarns. The belt shown in FIG. 3 can bemanufactured by stretching the yarns 8 and 9 in the circumferentialdirection and in the axial direction on a mandrel while defining aclearance between the same and the mandrel and casting the polyurethanelayer 7 from above. In other words, it is possible to manufacture apapermaking belt formed by integrating the reinforcing substrateemploying the reinforcing yarns 8 and 9 and the polyurethane layer 7with each other so that the reinforcing substrate employing thereinforcing yarns 8 and 9 is embedded in the polyurethane layer 7.

[0042] Each of FIGS. 4 to 6 shows an example prepared by forming thepolyurethane layer 7 of the belt shown in FIG. 2 by two layers.

[0043] Referring to FIG. 4, a fabric base 6 is embedded in apolyurethane layer 10 forming an inner peripheral surface and furthercovered with a polyurethane layer 11 forming an outer peripheralsurface, to be integrated. In order to manufacture the belt shown inFIG. 4, the relatively open weave fabric base 6 is arranged on a mandrelwhile defining a clearance between the same and the mandrel and thepolyurethane layer 10 is cast from above, after the method offabricating the belt shown in FIG. 2. The polyurethane layer 10 is socast from above as to form a polyurethane layer consisting of the fabricbase 6 and the polyurethane layer 10 integrated with each other so thatthe fabric base 6 is embedded in the polyurethane layer 10. The belt canbe manufactured by further coating this polyurethane layer with thepolyurethane layer 11 forming the outer peripheral surface.

[0044] The belt shown in FIG. 5 is manufactured by integrating a fabricbase 6 and two polyurethane layers 12 and 13 with each other byimpregnating and covering both surfaces of the fabric base 6 with thepolyurethane layers 12 and 13, for obtaining a papermaking belt havingthe fabric base 6 embedded in the polyurethane layers 12 and 13. Inorder to manufacture this belt, the fabric base 6 turned inside out iscoated with the polyurethane layer 12 forming an inner peripheralsurface, the fabric base is then turned inside out, and coated with thepolyurethane layer 13 forming an outer peripheral surface. In this case,the fabric base 6 serving as a reinforcing substrate is prepared from afine weave material allowing no passage of liquid polyurethane. Amulti-woven fabric base having permeability of 200 to 20 cm³/cm²·s canbe used as the fine weave fabric base.

[0045] As another manufacturing method, the polyurethane layer 12forming the inner peripheral surface is molded on a mandrel andthereafter the fabric base 6 is wound on the surface and further coatedwith the polyurethane layer 13 forming the outer peripheral surface,thereby obtaining a papermaking belt formed by integrating the fabricbase 6 and the polyurethane layers 12 and 13 with each other so that thefabric base 6 is embedded in the polyurethane layers 12 and 13.

[0046] Referring to FIG. 6, the lower portion of a polyurethane layer 15forming an outer peripheral surface having a fabric base 6 embeddedtherein is covered with a polyurethane layer 14 forming an innerperipheral surface, to be integrated. In order to manufacture this belt,the polyurethane layer 15 forming the outer peripheral surface havingthe fabric base embedded therein is molded after the method ofmanufacturing the belt shown in FIG. 2, and the inner peripheral surfacethereof is coated with the polyurethane layer 14 forming the innerperipheral surface later. As another method, the polyurethane layer 14forming the inner peripheral surface is previously molded on a mandrel,and the polyurethane layer 15 forming the outer peripheral surfacehaving the fabric base embedded therein is molded thereon after themethod of fabricating the belt shown in FIG. 2.

[0047] FIGS. 7 to 9 show examples, corresponding to the examples shownin FIGS. 4 to 6 respectively, employing reinforcing yarns 8 and 9 asreinforcing substrates in place of the fabric base 6.

[0048] In order to manufacture the belt shown in FIG. 7, a polyurethanelayer 16 forming an inner peripheral surface having the reinforcingyarns 8 and 9 embedded therein may be molded after the method ofmanufacturing the belt shown in FIG. 3, to be coated with a polyurethanelayer 17 forming an outer peripheral surface.

[0049] In order to manufacture the belt shown in FIG. 8, a polyurethanelayer 18 forming an inner peripheral surface may be molded on a mandrelfor thereafter winding yarns 8 and 9 in the circumferential directionand in the axial direction on the surface and further coating the samewith a polyurethane layer 19 forming an outer peripheral surface.

[0050] In order to manufacture the belt shown in FIG. 9, a polyurethanelayer 21 forming an outer peripheral surface having reinforcing yarns 8and 9 embedded therein is molded after the method of manufacturing thebelt shown in FIG. 3 and the inner peripheral surface thereof is coatedwith a polyurethane layer 20 forming an inner peripheral surface later.As another method, the polyurethane layer 20 forming the innerperipheral surface is previously molded on a mandrel, for thereaftermolding the polyurethane layer 21 forming the outer peripheral surfacehaving the reinforcing yarns 8 and 9 embedded therein after the methodof manufacturing the belt shown in FIG. 3.

[0051] The outer peripheral surface of each of the belts shown in FIGS.2 to 9 is formed by a polyurethane layer. The polyurethane layer 7, 11,13, 15, 17, 19 or 21 forming the outer peripheral surface is made of acomposition containing a urethane prepolymer having isocyanate groups(NCO) on ends and a hardener having active hydrogen groups (H) on ends.The urethane prepolymer is obtained by reacting polyol and a phenyleneisocyanate derivative with each other.

[0052] The polyol for obtaining the urethane prepolymer for thepolyurethane layer 7, 11, 13, 15, 17, 19 or 21 forming the outerperipheral surface is selected from polyether polyol and polyesterpolyol. Polyethylene glycol (PEG), polypropylene glycol (PPG),polytetramethylene glycol (PTMG) or the like can be listed as polyetherpolyol, for example. Polycaprolactone ester, polycarbonate, polyethyleneadipate, polybutylene adipate, polyhexene adipate or the like can belisted as polyester polyol. These can be individually employed or atleast two of these can be mixed or polymerized with each other, and amodified body of any of these can also be employed.

[0053] Tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI),m-xylene diisocyanate (m-XDI), naphthalene diisocyanate (NDI) or thelike can be listed as the phenylene isocyanate derivative for obtainingthe urethane prepolymer for the polyurethane layer 7, 11, 13, 15, 17, 19or 21 forming the outer peripheral surface, for example. These can beindividually employed or at least two of these can be mixed with eachother.

[0054] In general, a polyol, aromatic diol or aromatic diamine hardeneris used as the hardener for the polyurethane layer 7, 11, 13, 15, 17, 19or 21 forming the outer peripheral surface, It is possible to usepolytetramethylene glycol (PTMG), polypropylene glycol (PPG) or the likeas the polyol hardener. Hydroqhinone di(β-hydroxyethyl) ether (HQEE) orthe like can be used as the aromatic diol hardener.4,4′-methylene-bis-(2-chloroaniline) (MOCA),trimethylene-bis(4-aminobenzoate) (CUA-4), diethyltoluenediamine(DETDA), dimethylthiotoluenediamine (DMTDA) or the like can be used asthe aromatic diamine hardener. As a characteristic of the presentinvention, it is preferable to use a hardener containingdimethylthiotoluenediamine, a kind of aromatic diamine hardener, amongthese. 3,5-dimethylthio-2,4-toluenediamine expressed in the followingformula 1 can be used as dimethylthiotoluenediamine:

[0055] 3,5-dimethylthio-2,6-toluenediamine expressed in the followingformula 2 can be used as dimethylthiotoluenediamine:

[0056] 3,5-dimethylthio-2,4-toluenediamine or3,5-dimethylthio-2,6-toluenediamine can be employed individually or as amixture. A mixture of 3,5-dimethylthio-2,4-toluenediamine and3,5-dimethylthio-2,6-toluenediamine, put on the market as “ETHACURE 300”from ALBEMARLE Corporation, can be listed as a particularly preferablehardener.

[0057] When the hardener for the polyurethane layer 7, 11, 13, 15, 17,19 or 21 forming the outer peripheral surface contains theaforementioned dimethylthiotoluenediamine, one or two types of hardenersof polyol, aromatic diol, aromatic diamine and the like may be mixedinto the same. The content of the aforementioneddimethylthiotoluenediamine in the hardener for the polyurethane layer 7,11, 13, 15, 17, 19 or 21 forming the outer peripheral surface preferablyoccupies at least 50% of the number of active hydrogen groups (H) of thehardener. When the hardener for the polyurethane layer 7, 11, 13, 15,17, 19 or 21 forming the outer peripheral surface containsdimethylthiotoluenediamine, the polyurethane layer 7, 11, 13, 15, 17, 19or 21 forming the outer peripheral surface of the belt can be inhibitedfrom cracking.

[0058] From another point of view, the urethane prepolymer and thehardener are mixed with each other in a ratio setting the value of theequivalent ratio (H/NCO) between the active hydrogen groups (H) of thehardener and the isocyanate groups (NCO) of the urethane prepolymer to1<H/NCO<1.15 for the polyurethane layer 7, 11, 13, 15, 17, 19 or 21forming the outer peripheral surface. Even if a small crack occurs onthe polyurethane layer 7, 11, 13, 15, 17, 19 or 21 forming the outerperipheral surface of the belt, the crack can be suppressed from growinginto a large crack due to this structure. In the polyurethane layer 7,11, 13, 15, 17, 19 or 21 forming the outer peripheral surface, theurethane prepolymer and the hardener can also be mixed with each otherin a ratio setting the value of the equivalent ratio (H/NCO) to1.01≦H/NCO≦1.14, so that, even if a small crack occurs, the crack can bemore accurately suppressed from growing into a large crack in this case.If the value of the equivalent ratio H/NCO in the polyurethane layer 7,11, 13, 15, 17, 19 or 21 forming the outer peripheral surface is notmore than 1, a crack tends to grow into a large crack. If the value ofthe equivalent ratio H/NCO in the polyurethane layer 7, 11, 13, 15, 17,19 or 21 forming the outer peripheral surface is in excess of 1.15, onthe other hand, the polyurethane layer is so fragile that a crackreadily occurs.

[0059] When a hardener containing dimethylthiotoluenediamine is used asthe hardener for the polyurethane layer 7, 11, 13, 15, 17, 19 or 21forming the outer peripheral surface while the urethane prepolymer andthe hardener are mixed with each other in a ratio setting the value ofthe equivalent ratio (H/NCO) between the active hydrogen groups (H) ofthe hardener and the isocyanate groups (NCO) of the urethane prepolymerto 1<H/NCO<1.15, the polyurethane layer 7, 11, 13, 15, 17, 19 or 21forming the outer peripheral surface of the belt can be inhibited fromcracking, and even if a small crack occurs, this crack can be suppressedfrom growing into a large crack. The content ofdimethylthiotoluenediamine in the hardener preferably occupies at least50% of the number of active hydrogen groups (H) in the hardener. Theurethane prepolymer and the hardener are preferably mixed with eachother in the ratio setting the value of the equivalent ratio (H/NCO) to1.01≦H/NCO≦1.14.

[0060] In order to improve the dehydration efficiency, a number ofgrooves 22 are preferably formed on the outer peripheral surface of thebelt shown in each of FIGS. 2 to 9 along the traveling direction of thebelt, as shown in FIG. 10. According to the present invention, crackingon the papermaking belt and growth of the crack can be suppressed due tothe aforementioned structure, whereby cracking from the bottom edges andthe top edges of the grooves 22 can be suppressed also when the grooves22 are formed on the outer peripheral surface of the belt. A number ofblind holes may be provided on the outer peripheral surface of the beltin place of or along with the grooves 22.

[0061] While only a single polyurethane layer is provided in addition tothe polyurethane layer 7, 11, 13, 15, 17, 19 or 21 forming the outerperipheral surface in each of FIGS. 4 to 9, the polyurethane layer 10,12, 14, 16, 18 or 20 other than the polyurethane layer forming the outerperipheral surface may be divided into a plurality of layers.

[0062] While each belt has an endless shape as a whole, each individuallayer may not necessarily be in the form of a layer. For example, acertain polyurethane layer may be present only on a part along the widthof the belt.

[0063] The reinforcing substrate 6, 8 or 9 may be embedded in any singlepolyurethane layer, or may extend over a plurality of arbitrarypolyurethane layers.

[0064] Each of the belts shown in FIGS. 4 to 9 includes the innerpolyurethane layer 10, 12, 14, 16, 18 or 20 and the outer polyurethanelayer 11, 13, 15, 17, 19 or 21.

[0065] The inner polyurethane layer 10, 12, 14, 16, 18 or 20 is made ofa composition containing a urethane prepolymer having isocyanate groups(NCO) on ends and a hardener having active hydrogen groups (H) on ends,similarly to the outer polyurethane layer (the said polyurethane layerforming the outer peripheral surface) 11, 13,15, 17, 19 or 21. Theurethane prepolymer is obtained by reacting polyol and a phenyleneisocyanate derivative with each other.

[0066] The polyol and the phenylene isocyanate derivative for obtainingthe urethane prepolymer for the inner polyurethane layer 10, 12, 14, 16,18 or 20 are similar to those described with reference to the outerpolyurethane layer (the said polyurethane layer forming the outerperipheral surface) 11, 13, 15, 17, 19 or 21. One of or a mixture of atleast two of polyol, aromatic diol and aromatic diamine hardenersgenerally employable as hardeners for polyurethane can be employed asthe hardener for the inner polyurethane layer 10, 12, 14, 16, 18 or 20.

[0067] In the composition forming the inner polyurethane layer 10, 12,14, 16, 18 or 20, the urethane prepolymer and the hardener are mixedwith each other in a ratio setting the equivalent ratio (H/NCO) betweenthe active hydrogen groups (H) of the hardener and the isocyanate groups(NCO) of the urethane prepolymer to 0.85≦H/NCO<1. In the compositionforming the inner polyurethane layer 10, 12, 14, 16, 18 or 20, theurethane prepolymer and the hardener can also be mixed with each otherin a ratio setting the value of the equivalent ratio (H/NCO) to0.85≦H/NCO≦0.99.

[0068] In the composition forming the outer polyurethane layer 11, 13,15, 17, 19 or 21, on the other hand, the urethane prepolymer and thehardener are mixed with each other in a ratio setting the value of theequivalent ratio (H/NCO) between the active hydrogen groups (H) of thehardener and the isocyanate groups (NCO) of the urethane prepolymer to1<H/NCO<1.15. The urethane prepolymer and the hardener are preferablymixed with each other in a ratio setting the value of the equivalentratio (H/NCO) to 1.01≦H/NCO≦1.14.

[0069] When the mixing ratio for the hardener and the urethaneprepolymer is set to 0.85≦H/NCO<1 for the inner polyurethane layer 10,12, 14, 16, 18 or 20 and set to 1<H/NCO<1.15 for the outer polyurethanelayer 11, 13, 15, 17, 19 or 21, adhesion between the inner polyurethanelayer 10, 12, 14, 16, 18 or 20 and the outer polyurethane layer 11, 13,15, 17, 19 or 21 is improved and delamination can be suppressed. Thereason for this is conceivably because the equivalent ratio for theinner polyurethane layer 10, 12, 14, 16, 18 or 20 is set to 0.85≦H/NCO<1so that NCO groups stoichiometrically remain and these residualisocyanate groups (NCO) react with surplus active hydrogen groups (H) ofthe outer polyurethane layer 11, 13, 15, 17, 19 or 21 having theequivalent ratio set to 1<H/NCO<1.15, to strongly adhere to andintegrate with the same. When the mixing ratio of the hardener and theurethane prepolymer is set to 0.85≦H/NCO≦0.99 for the inner polyurethanelayer 10, 12, 14, 16, 18 or 20 and to 1.01≦H/NCO≦1.14 for the outerpolyurethane layer, 11, 13, 15, 17, 19 or 21, delamination can be morepreferably suppressed.

[0070] In the belt shown in FIG. 5, the fabric base 6 is impregnated andcovered with two polyurethane layers from both surfaces, to beintegrated therewith. The adhesive surface between the innerpolyurethane layer 12 and the outer polyurethane layer 13 is present inthe fabric base 6. Therefore, an anchor effect is attained between thefabric base 6 and the polyurethane layers 12 and 13 in addition to theadhesion, whereby strong adhesion is attained so that the belt can beprevented from delamination.

[0071] As a preferred mode of the belt shown in FIG. 5, the urethaneprepolymer forming the inner polyurethane layer 12 contains an MDIurethane prepolymer obtained by reacting polyol and diphenylmethanediisocyanate (MDI) with each other while the urethane prepolymer formingthe outer polyurethane layer 13 contains a TDI urethane prepolymerobtained by reacting polyol and tolylene diisocyanate (TDI) with eachother.

[0072] The polyurethane using the MID prepolymer relatively quicklyreacts and has a short hardening time. Therefore, the prepolymer formingthe inner polyurethane layer 12 is mainly composed of the MDI prepolymerso that the polyurethane layer 12 can be prevented from passing throughthe fabric base 6 toward the opposite surface when the fabric base 6 iscoated with the inner polyurethane layer 12 and the position ofimpregnation can be stopped in the fabric base 6 in the stage ofmanufacturing the belt. On the other hand, the polyurethane using theTDI prepolymer relatively slowly reacts and has a long hardening time.Therefore, the prepolymer forming the outer polyurethane layer 13 ismainly composed of the TDI prepolymer so that the outer polyurethanelayer 13 can be sufficiently infiltrated into the position impregnatedwith the inner polyurethane layer 12. Thus, the adhesive surface betweenthe inner polyurethane layer 12 and the outer polyurethane layer 13 canbe formed in the fabric base 6.

[0073] In the inner polyurethane layer 12, polyol preferably occupies atleast 50 wt. % of the hardener with respect to the MDI urethaneprepolymer. In this case, the hardening time of the polyurethane can bereadily adjusted for adjusting the position for impregnating the fabricbase 6. A polyol hardener is selected from polyether polyol andpolyester polyol. Polyethylene glycol (PEG), polypropylene glycol (PPG),polytetramethylene glycol (PTMG) or the like can be listed as polyetherpolyol, for example. Polycaprolactone ester, polycarbonate, polyethyleneadipate, polybutylene adipate, polyhexene adipate or the like can belisted as polyester polyol. These can be individually employed or atleast two of these can be mixed or polymerized with each other, and amodified body thereof can also be employed. The hardener for the innerpolyurethane layer 12 may contain at least 50 wt. % of polyol, and maybe mixed with one or at least two types of aromatic diol or aromaticdiamine hardeners.

[0074]FIG. 11 shows a more preferred mode of the belt shown in FIG. 5.The belt shown in FIG. 11 employs a fabric base 23 consisting ofmulti-woven fabric in the belt shown in FIG. 5. This fabric base 23preferably includes a number of voids, in order to improve the degree ofimpregnation of polyurethane. When the multi-woven fabric base 23 isemployed, the following effects can be attained in addition to theexcellent strength of the fabric base 23 itself: When the fabric base 23consisting of multi-woven fabric is employed, polyurethane cansufficiently infiltrate into the fabric base 23 for forming an adhesivesurface between an inner polyurethane layer 24 and an outer polyurethanelayer 25 in the fabric base 23. Further, a sufficient anchor effect canbe attained between the polyurethane layers 24 and 25 and the fabricbase 23. Therefore, strong adhesion is attained between the innerpolyurethane layer 24 and the outer polyurethane layer 25, forpreventing the belt from delamination. Quadruple layer weaving, triplelayer weaving or the like can be listed as exemplary multi-weaving.Polyurethane used for the belt shown in FIG. 11 is similar to that shownin FIG. 5. On the outer peripheral surface of the belt shown in FIG. 11,a number of grooves 26 are formed along the traveling direction in orderto improve dehydration efficiency.

[0075] In each of the belts shown in FIGS. 4 to 9 and 11, the outerpolyurethane layer 11, 13, 15, 17, 19, 21 or 25 adheres to the outerperipheral surface of the inner polyurethane layer 10, 12, 14, 16, 18,20 or 24, and forms the outer peripheral surface of the papermakingbelt. In this belt, the outer polyurethane layer 11, 13, 15, 17, 19, 21or 25 is preferably made of a composition containing a urethaneprepolymer having isocyanate groups on ends and a hardener mainlycomposed of dimethylthiotoluenediamine as described above. When thehardener for the outer polyurethane layer 11, 13, 15, 17, 19, 21 or 25including the outer peripheral surface is mainly composed ofdimethylthiotoluenediamine, the outer peripheral surface of the belt canbe inhibited from cracking as described above.

[0076] In each of the belts shown in FIGS. 4 to 9 and 11, as hereinabovedescribed, the urethane prepolymer and the hardener are mixed with eachother in the ratio setting the value of the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 1<H/NCO<1.15 inthe composition forming the outer polyurethane layer 11, 13, 15, 17, 19,21 or 25 including the outer peripheral surface. Even if a small crackoccurs in the polyurethane layer 11, 13, 15, 17, 19, 21 or 25 formingthe outer peripheral surface of the belt, therefore, the crack can beinhibited from growing into a large crack. Even if a small crack occurs,the crack can be more accurately prevented from growing into a largecrack when the urethane prepolymer and the hardener are mixed with eachother in a ratio setting the value of the equivalent ratio (H/NCO) to1.01≦H/NCO≦1.14.

[0077]FIG. 12 shows another embodiment. In a belt shown in FIG. 12, afabric base consisting of multi-woven fabric is impregnated and coveredwith an inner polyurethane layer 27 and an outer polyurethane layer 28from both surfaces, and the outer polyurethane layer 28 is furthercovered and integrated with a polyurethane layer 29 forming an outerperipheral surface. The adhesive surface between the inner polyurethanelayer 27 and the outer polyurethane layer 28 is present in the fabricbase 23. One or a plurality of polyurethane layers may further be formedbetween the outer polyurethane layer 28 and the polyurethane layer 29forming the outer peripheral surface. In the example shown in FIG. 12,the position of the interface between the outer polyurethane layer 28and the polyurethane layer 29 forming the outer peripheral surface isflush with the surface of the fabric base 23. However, the position ofthe interface between the outer polyurethane layer 28 and thepolyurethane layer 29 forming the outer peripheral surface is notrestricted to this but may vertically deviate from the surface of thefabric base 23. Also on the outer peripheral surface of the belt shownin FIG. 12, a number of grooves 26 are formed along the travelingdirection for improving dehydration efficiency.

[0078] In the belt shown in FIG. 12, the polyurethane layer 29 formingthe outer peripheral surface is made of a composition containing aurethane prepolymer having isocyanate groups on ends and a hardenermainly composed of dimethylthiotoluenediamine. Also in this example, theouter peripheral surface of the belt can be inhibited from cracking bypreparing the main component of the hardener for the polyurethane layer29 forming the outer peripheral surface from dimethylthiotoluenediamine,as hereinabove described.

[0079] In the belt shown in FIG. 12, the polyurethane layer 29 formingthe outer peripheral surface is preferably prepared by mixing theurethane prepolymer and the hardener with each other in the ratiosetting the value of the equivalent ratio (H/NCO) between the activehydrogen groups (H) of the hardener and the isocyanate groups (NCO) ofthe urethane prepolymer to 1<H/NCO<1.15. Even if a small crack occurs inthe polyurethane layer 29 forming the outer peripheral surface of thebelt, the crack can be inhibited from growing into a large crack due tothis structure. The urethane prepolymer and the hardener are morepreferably mixed with each other in a ratio setting the value of theequivalent ratio (H/NCO) to 1.01≦H/NCO≦1.14.

[0080] The belt shown in FIG. 12 can be provided as a belt containing nobubbles causing breakage or delamination due to the presence of theouter polyurethane layer 28 between the inner polyurethane layer 27 andthe polyurethane layer 29 forming the outer peripheral surface.

[0081] In order to manufacture the belt, the fabric base 23 is firstturned inside out. The surface defining the back surface of the fabricbase 23 is coated with the inner polyurethane layer 27 for infiltratingpolyurethane into an intermediate portion of the fabric base 23. Then,the fabric base 23 is reversed and coated with the outer polyurethanelayer 28 to fill up the remaining portion of the fabric base 23 from thesurface. The outer polyurethane layer 28 is further coated with thepolyurethane layer 29 forming the outer peripheral surface. Thus, airremaining in the fabric base 23 can be expelled when the same is coatedwith the outer polyurethane layer 28. Therefore, a belt containing nobubbles can be obtained.

[0082] In the belt shown in FIG. 12, the outer polyurethane layer 28 maybe made of the same composition as the inner polyurethane layer 27 orthe polyurethane layer 29 forming the outer peripheral surface, or maybe made of another composition.

[0083] While the example shown in FIG. 12 includes three polyurethanelayers, i.e., the inner polyurethane layer 27, the outer polyurethanelayer 28 and the polyurethane layer 29 forming the outer peripheralsurface, the number of such polyurethane layers may be only one, two orat least four. When the outer polyurethane layer 28 is divided into aplurality of thin layers for coating the fabric base 23, for example,air contained in the fabric base 23 can be more effectively expelled.The fabric base 23 can also be coated with the inner polyurethane layer27 a plurality of times.

[0084] Throughout the present invention, polyurethane is preferablyhardened at a temperature of 120° C. to 140° C. Thus, crack resistanceas well as crack propagation resistance of the belt are improved.

[0085] A method of manufacturing the belt shown in FIG. 11 is nowdescribed. As a first step, the fabric base 23 consisting of endlessmulti-woven fabric is turned inside out. The surface defining the backsurface of the fabric base is coated with the inner polyurethane layer24 for infiltrating polyurethane into an intermediate portion of thefabric base 23. This polyurethane is a liquid mixture, containing aurethane prepolymer having isocyanate groups on ends and a hardenerhaving active hydrogen groups on ends, prepared by mixing the urethaneprepolymer and the hardener with each other in a ratio setting the valueof the equivalent ratio (H/NCO) between the active hydrogen groups (H)of the hardener and the isocyanate groups (NCO) of the urethaneprepolymer to 0.85≦H/NCO<1. The liquid mixture is preferably prepared bymixing the urethane prepolymer and the hardener with each other in aratio setting the value of the equivalent ratio (H/NCO) to0.85≦H/NCO≦0.99. The polyurethane forming the coat is hardened at atemperature of 70° C. to 100° C.

[0086] Then, the fabric base 23 is reversed and coated with the outerpolyurethane layer 25 from the surface side to fill up the remainingportion of the fabric base 23 as a second step. This polyurethane is aliquid mixture, containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on ends,prepared by mixing the urethane prepolymer and the hardener with eachother in a ratio setting the value of the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 1<H/NCO<1.15. Theliquid mixture is preferably prepared by mixing the urethane prepolymerand the hardener with each other in a ratio setting the value of theequivalent ratio (H/NCO) to 1.01≦H/NCO≦1.14.

[0087] Then, the whole is heated to a temperature of 120° C. to 140° C.for hardening the liquid mixture applied onto the outer peripheralsurface of the inner polyurethane layer 24 and forming the outerpolyurethane layer 25 while bonding and integrating the innerpolyurethane layer 24 and the outer polyurethane layer 25 to and witheach other as a third step.

[0088] The belt shown in FIG. 11 can be obtained by thereafter formingthe number of grooves 26 on the outer peripheral surface of the beltalong the traveling direction.

[0089] According to this method, polyurethane having the composition of0.85≦H/NCO<1 stoichiometrically containing remaining NCO groups issemi-hardened at the relatively low temperature of 70° C. to 100° C. inthe first step. In the second step, the semi-hardened inner polyurethanelayer 24 is coated with the outer polyurethane layer 25 having thecomposition of 1<H/NCO<1.15 containing the hardener in a large quantity.Then, the whole is heated to the relatively high temperature of 120° C.to 140° C. and hardened in the third step. Therefore, the adhesionbetween the inner polyurethane layer 24 and the outer polyurethane layer25 is improved so that delamination can be suppressed.

[0090] The fabric base 23 consisting of multi-woven fabric forms thereinforcing substrate, whereby the adhesive surface between the innerpolyurethane layer 24 and the outer polyurethane layer 25 can be readilylocated in the fabric base 23. In addition to the adhesion, an anchoreffect can be attained between the fabric base 23 and the polyurethanelayers 24 and 25 due to the adhesive surface located in the fabric base23. Therefore, strong adhesion can be attained and the belt can beprevented from delamination.

[0091] When employing an open weave fabric base capable of sufficientlypassing liquid polyurethane therethrough or the yarns 8 and 9 shown inFIG. 8 for the reinforcing substrate in place of the multi-woven fabricbase 23 as a modification of the aforementioned manufacturing method,the belt can be manufactured as follows: Describing the manufacturingmethod with reference to the belt shown in FIG. 8, a mandrel is coatedwith a liquid mixture containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on endsprepared by mixing the urethane prepolymer and the hardener with eachother in a ratio setting the value of the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 0.85≦H/NCO<1 andthe liquid mixture is hardened at a temperature of 70° C. to 100° C. forforming the inner polyurethane layer 18. The urethane prepolymer and thehardener are more preferably mixed with each other in a ratio settingthe value of the equivalent ratio (H/NCO) to 0.85≦H/NCO≦0.99. Then, theyarn 9 in the CMD and the yarn 8 in the MD are wound on the outerperipheral surface of the inner polyurethane layer as reinforcingsubstrates. As a second step, the reinforcing substrates 8 and 9 arecoated with a liquid mixture containing a urethane prepolymer havingisocyanate groups on ends and a hardener having active hydrogen groupson ends, prepared by mixing the urethane prepolymer and the hardenerwith each other in a ratio setting the value of the equivalent ratio(H/NCO) between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 1<H/NCO<1.15. Theliquid mixture is more preferably prepared by mixing the urethaneprepolymer and the hardener with each other in a ratio setting the valueof the equivalent ratio (H/NCO) to 1.01≦H/NCO≦1.14. Then, the whole isheated to a temperature of 120° C. to 140° C. for hardening the liquidmixture applied onto the outer peripheral surface of the innerpolyurethane layer 18 and forming the outer polyurethane layer 19 whilebonding and integrating the inner polyurethane layer 18 and the outerpolyurethane layer 19 to and with each other as a third step.

[0092] In order to manufacture the belt shown in FIG. 12, a step offurther coating the outer polyurethane layer 28 with the polyurethanelayer 29 forming the outer peripheral surface may be added between thesecond and third steps in the aforementioned method of manufacturing thebelt shown in FIG. 11.

[0093] While the above description has been made with reference to abelt for shoe pressing, the present invention is also applicable to acalender belt and a sheet transfer belt. The calender belt and the sheettransfer belt are not formed with grooves on the surfaces thereof ingeneral.

EXAMPLE

[0094] As Example, the papermaking belt shown in FIG. 12 wasmanufactured in the following procedure: An endless fabric base 23consisting of quadruple layer woven fabric was prepared as thereinforcing substrate. This fabric base was 2.3 mm in thickness, andcontained voids. As to the structure of the fabric base, warps of the MDconsisted of four layers of polyester monofilaments 30 of 0.35 mm indiameter, polyester multifilaments 31 of 3000 d, polyester monofilaments32 of 0.35 mm in diameter and nylon monofilaments 33 of 0.35 mm indiameter successively from the surface side and wefts of the CMDconsisted of polyester monofilaments 34 of 0.40 mm in diameter, as shownin FIG. 13. The number of the warps was 68/inch, and the number of thewefts was 56/inch.

[0095] As the material for the inner polyurethane layer 27, 100 parts byweight of a urethane prepolymer (PTMG/MDI: NCO %=5%) and 27.4 parts byweight of a hardener (prepared by blending PTMG and ETHACURE 300 in aratio of 65/35: equivalent=250) were individually defoamed andthereafter mixed with each other (H/NCO=0.92). The term NCO % stands forthe weight percentage of the isocyanate groups contained in the urethaneprepolymer. The surface of the fabric base 23 turned inside out wascoated with this mixture, which in turn was heated under a temperaturecondition of 80° C. for 10 hours. The fabric base 23 was impregnatedwith the inner polyurethane layer 27 up to 50% of the thickness.

[0096] Then, the polyurethane layer 27 coating the fabric base 23 wascut and ground so that the thickness from the surface of the fabric base23 was 1.0 mm. Thereafter the fabric base 23 was reversed to direct thecoated surface inward.

[0097] Then, 100 parts by weight of a urethane prepolymer (PTMG/TDI: NCO%=5%) and 13.8 parts by weight of a hardener (ETHACURE 300:equivalent=107) were individually defoamed and thereafter mixed witheach other as the material for forming the outer polyurethane layer 28,for coating the other surface of the fabric base 23 with this mixturewhile impregnating the same with the mixture up to the surfaceimpregnated with the inner polyurethane layer 27. The coated surface wassmoothed with a doctor blade to be substantially flush with the positionof the surface of the fabric base 23.

[0098] Further, the outer polyurethane layer 28 was coated with the samematerial as the said outer polyurethane layer 28 as the polyurethanelayer 29 forming the outer peripheral surface. Thereafter heating wasperformed under a temperature condition of 120° C. for 16 hours forbonding and integrating the inner polyurethane layer 27, the outerpolyurethane layer 28, the polyurethane layer 29 forming the outerperipheral surface and the fabric base 23 to and with each other.

[0099] Further, the surface of the belt was cut and ground so that thethickness of the polyurethane layer 29 forming the outer peripheralsurface was 1.5 mm. In addition, a number of grooves 26 were formed onthe outer surface of the belt along the traveling direction at a groovewidth of 0.8 mm, a depth of 0.8 mm and a pitch of 2.54 mm. The totalthickness and the JIS-A surface hardness of the obtained belt were 4.8mm and 90° respectively.

[0100] Then, samples 1 to 6 each having a structure shown in FIG. 14were prepared as follows: A fabric base 23 consisting of quadruple layerwoven fabric identical to that employed for the aforementioned fabricbase was prepared as the reinforcing substrate. As a polyurethane layer35 forming an inner surface, 100 parts by weight of a urethaneprepolymer (PTMG/MDI: NCO %=5%) and 27.4 parts by weight of a hardener(obtained by blending PTMG and ETHACURE 300 in a ratio of 65/35:equivalent=250) were individually defoamed and thereafter mixed witheach other (H/NCO=0.92). The back surface of the fabric base 23 wascoated with this mixture, which in turn was heated under a temperaturecondition of 80° C. for 10 hours. The fabric base 23 was impregnatedwith the polyurethane layer 35 forming the inner surface up to 50% ofthe thickness. Then, the polyurethane layer 35 coating the fabric base23 was cut and ground so that the thickness from the surface of thefabric base 23 was 1.0 mm.

[0101] Then, urethane prepolymers HIPRENE L-100 and HIPRENE L-167 (eachPTMG/TDI prepolymer: by Mitsui Chemicals) and hardeners ETHACURE 300 andMOCA were used as the materials for a polyurethane layer 36 forming anouter surface. Each urethane prepolymer and each hardener wereindividually defoamed and thereafter mixed with each other in eachcomposition shown in Table 1, and the fabric base 23 was impregnated andcoated with the mixture up to the surface impregnated with thepolyurethane layer 35 forming the inner surface from a surface definingthe outer surface of the fabric base 23.

[0102] Thereafter heating was performed under a temperature condition of120° C. for 16 hours, for bonding and integrating the polyurethane layer35 forming the inner surface, the polyurethane layer 36 forming theouter surface and the fabric base 23 to and with each other. Further,the surface of the belt was cut and ground so that the thickness of thepolyurethane layer 36 forming the outer surface from the surface of thefabric base 23 was 1.5 mm, for obtaining each of the samples 1 to 6.

[0103] A test piece of 20 mm in width and 420 mm in length was obtainedfrom each of the samples 1 to 6. As shown in FIG. 15, both longitudinalends of each test piece 37 were gripped with gripping members 38 forbringing a metal round bar 39 of 25 mm in diameter having a smoothsurface into contact with the inner side of an intermediate portion andapplying tension T. The tension T was set to 9.8 kN/m. The test piece 37was repetitively reciprocated with a width of 10 cm while keeping thetension T and supplying lubricating oil between the inner surface of thetest piece 37 and the round bar 39 from a nozzle 40. According to thismethod, sliding was repeated between the inner surface and the round bar39 while applying the tension T to the test piece 37. The number oftimes of reciprocation up to cracking on the surface of the test piece37 was measured as an endurance limit. Table 1 and FIG. 16 show theresults. Table 1 also shows the hardness of each of the samples 1 to 6.Referring to Table 1, the quantity of the hardener indicates the numberof parts by weight of the hardener with respect to 100 parts by weightof the prepolymer. TABLE 1 Polyurethane forming Outer Surface EnduranceEquivalent Limit (× ten Prepolymer Hardener Ratio Hardness thousandL-100 L-167 NCO % Type Quantity Equivalent (H/NCO) (JIS A) times) Sample1 100 0 4.2 DMTDA 11.1 107 1.04 89 2250 Sample 2 50 50 5.3 DMTDA 14.0107 1.04 91 750 Sample 3 0 100 6.4 DMTDA 17.0 107 1.04 94 250 Sample 4100 0 4.2 MOCA 13.9 133.6 1.04 90 90 Sample 5 50 50 5.3 MOCA 17.5 133.61.04 92 30 Sample 6 0 100 6.4 MOCA 21.1 133.6 1.04 95 10

[0104] Each of samples 7 to 30 was prepared as follows: The reinforcingsubstrate 23 and the polyurethane layer 35 forming the inner surfacewere similar to those of the aforementioned samples 1 to 6. As to thematerials for the polyurethane layer 36 forming the outer surface, L-100and L-167 were used as the urethane prepolymers and ETHACURE 300 wasused as the hardener. Each urethane prepolymer and the hardenet wereindividually defoamed and thereafter mixed with each other in eachcomposition varying in H/NCO equivalent ratio as shown in Table 2, andthe surface forming the outer side of the fabric base 23 was impregnatedand coated with this mixture up to the surface impregnated with thepolyurethane layer 35 forming the inner surface.

[0105] Thereafter heating was performed under a temperature condition of120° C. for 16 hours, for bonding and integrating the polyurethane layer35 forming the inner surface, the polyurethane layer 36 forming theouter surface and the fabric base 23 to and with each other. Further,the surface of the belt was cut and ground so that the thickness of thepolyurethane layer 36 forming the outer surface from the surface of thefabric base 23 was 1.5 mm, for obtaining each of the samples 7 to 30.

[0106] As to each of the samples 7 to 30, crack propagation resistancewas tested with a de Mattia machine defined in JIS K6260 under thefollowing conditions: The test piece was set to 20 mm in width and 150mm in length. Reciprocating motion was made at a maximum distance of80.5 mm, a minimum distance of 38.5 mm and a motion distance of 42.0 mm.A notch was formed on the outer surface of an end of the test piece inthe width direction at the longitudinal center with a length of 3 mm anda depth of 1.5 mm. The test piece was bent 1000 times under theseconditions, for thereafter measuring the magnitude of cracking. Table 2shows the results in the item of crack propagation length. Further, eachof the samples 7 to 30 was reciprocated million times with a testershown in FIG. 15, for visually confirming whether or not the test piecewas cracked. Table 2 shows the results in the item of presence/absenceof cracking. Referring to Table 2, the quantity of the hardenerindicates the number of parts by weight of the hardener with respect to100 parts by weight of the prepolymer. TABLE 2 Polyurethane formingOuter Surface Presence/Absence of Equivalent Length of Progress CrackingPrepolymer Hardener Ratio Hardness of Cracking (million L-100 L-167 NCO% Type Quantity Equivalent (H/NCO) (JIS A) (mm/1000 bends)reciprocations) Sample 7 100 0 4.2 DMTDA 9.8 107 0.92 89 5 no Sample 8100 0 4.2 DMTDA 10.3 107 0.96 89 2.9 no Sample 9 100 0 4.2 DMTDA 10.7107 1 89 0.8 no Sample 10 100 0 4.2 DMTDA 10.8 107 1.01 89 0.5 no Sample11 100 0 4.2 DMTDA 11.1 107 1.04 89 0.2 no Sample 12 100 0 4.2 DMTDA11.6 107 1.08 89 0.01 no Sample 13 100 0 4.2 DMTDA 12.2 107 1.14 890.005 no Sample 14 100 0 4.2 DMTDA 12.3 107 1.15 89 0.001 yes Sample 1550 50 5.3 DMTDA 12.4 107 0.92 91 7 no Sample 16 50 50 5.3 DMTDA 13.0 1070.96 91 4.1 no Sample 17 50 50 5.3 DMTDA 13.5 107 1 91 1.1 no Sample 1850 50 5.3 DMTDA 13.6 107 1.01 91 0.7 no Sample 19 50 50 5.3 DMTDA 14.0107 1.04 91 0.4 no Sample 20 50 50 5.3 DMTDA 14.6 107 1.08 91 0.1 noSample 21 50 50 5.3 DMTDA 15.4 107 1.14 91 0.05 no Sample 22 50 50 5.3DMTDA 15.5 107 1.15 91 0.005 yes Sample 23 0 100 6.4 DMTDA 15.0 107 0.9294 9 no Sample 24 0 100 6.4 DMTDA 15.7 107 0.96 94 5.1 no Sample 25 0100 6.4 DMTDA 16.3 107 1 94 1.5 no Sample 26 0 100 6.4 DMTDA 16.5 1071.01 94 0.9 no Sample 27 0 100 6.4 DMTDA 17.0 107 1.04 94 0.6 no Sample28 0 100 6.4 DMTDA 17.6 107 1.08 94 0.3 no Sample 29 0 100 6.4 DMTDA18.6 107 1.14 94 0.1 no Sample 30 0 100 6.4 DMTDA 18.8 107 1.15 94 0.01yes

[0107] As understood from Table 2, it was possible to suppress the crackpropagation length to less than 1 mm in each sample having theequivalent ratio (H/NCO) in the polyurethane layer forming the outersurface greater than 1. The crack propagation length can be suppressedas the equivalent ratio (H/NCO) is increased. When the equivalent ratio(H/NCO) was increased to 1.15, however, cracking was observed in themillion-reciprocation test.

[0108] Then, the equivalent ratio (H/NCO) in the polyurethane layerforming the inner surface was varied for preparing each of samples 31 to36 having the structure shown in FIG. 14 as follows: The reinforcingsubstrate 23 was identical to those of the samples 1 to 30. Thepolyurethane layer 36 forming the outer surface was prepared byemploying L-167 as the urethane prepolymer and ETHACURE 300 as thehardener in the same mixing ratio as the aforementioned sample 27. As tothe polyurethane layer 35 forming the inner surface, the same materialsas those employed for the aforementioned sample 27, i.e., the urethaneprepolymer (PTMG/MDI: NCO %=5%) and the hardener (prepared by mixingPTMG and ETHACURE 300 in the ratio of 65/35: equivalent=250) wereemployed. However, the mixing ratio of the urethane prepolymer and thehardener for the polyurethane layer 35 forming the inner surface wasvaried for preparing the samples 31 to 36. The remaining manufacturingconditions and the thicknesses of the respective layers were setidentically to the samples 1 to 30.

[0109] A test piece of 20 mm in width and 420 mm in length was preparedfrom each of the samples 31 to 36, and subjected to a durability testwith the tester shown in FIG. 15 similarly to the samples 1 to 6.Evaluation was made by confirming a state after reciprocation for 2.5million times as to each sample. Table 3 shows the results. Referring toTable 3, the prepolymers and the hardeners are shown in parts by weight.TABLE 3 Polyurethane forming Inner Surface Polyurethane forming OuterSurface Equivalent Equivalent Prepolymer Hardener Ratio PrepolymerHardener Ratio State of Sample 1 2 (H/NCO) 3 4 (H/NCO) (2.5 milliontimes) Sample 31 100 30.4 1.02 100 17.0 1.04 boundary between layersimpregnated with resin separated Sample 32 100 29.8 1.00 100 17.0 1.04boundary between layers impregnated with resin partially separatedSample 33 100 29.2 0.98 100 17.0 1.04 no problem Sample 34 100 27.4 0.92100 17.0 1.04 no problem Sample 35 100 25.3 0.85 100 17.0 1.04 noproblem Sample 36 100 23.8 0.80 100 17.0 1.04 polyurethane layer forminginner surface slightly cracked

[0110] When the equivalent ratio H/NCO in the polyurethane layer formingthe inner surface was less than 0.85, the strength of the polyurethanelayer forming the inner surface was reduced to result in small cracks.When the equivalent ratio H/NCO in the polyurethane layer forming theinner surface exceeded 1, delamination was caused.

[0111] The equivalent ratio H/NCO in the polyurethane layer forming theouter surface is preferably set to 1<H/NCO<1.15. According to each ofthe samples 7 to 30 shown in Table 2, cracking readily spreads if theequivalent ratio H/NCO in the polyurethane layer forming the outersurface is not more than 1, while cracking readily occurs if theequivalent ratio is in excess of 1.15.

[0112] The embodiment and Example disclosed this time must be consideredillustrative in all points and not restrictive. The scope of the presentinvention is shown not by the above description but by the scope ofclaim for patent, and it is intended that all modifications in themeaning and range equivalent to the scope of claim for patent areincluded.

INDUSTRIAL APPLICABILITY

[0113] The papermaking belt according to the present invention, havingthe polyurethane layer forming the outer peripheral surface made of thecomposition containing the urethane prepolymer having isocyanate groupson ends and the hardener containing dimethylthiotoluenediamine, canprevent cracking. Further, the papermaking belt according to the presentinvention, having the polyurethane layer forming the outer peripheralsurface made of the composition containing the urethane prepolymerhaving isocyanate groups on ends and the hardener having active hydrogengroups on ends with the said composition prepared by mixing the saidurethane prepolymer and the said hardener with each other in the ratiosetting the value of the equivalent ratio (H/NCO) between the activehydrogen groups (H) of the said hardener and the isocyanate groups (NCO)of the said urethane prepolymer to 1<H/NCO<1.15, whereby, even if acrack occurs in the papermaking belt, this crack can be inhibited fromgrowing. In the papermaking belt according to the present invention, thecomposition forming the inner polyurethane layer is prepared by mixingthe urethane prepolymer and the hardener with each other in the ratiosetting the equivalent ratio (H/NCO) between the active hydrogen groups(H) of the hardener and the isocyanate groups (NCO) of the urethaneprepolymer to 0.85≦H/NCO<1 and the composition forming the outerpolyurethane layer is prepared by mixing the urethane prepolymer and thehardener with each other in the ratio setting the value of theequivalent ratio (H/NCO) to 1<H/NCO<1.15, whereby delamination can besuppressed between the reinforcing substrate and the polyurethanelayers.

1. A papermaking belt comprising a reinforcing substrate embedded in athermosetting polyurethane layer and having an outer peripheral surfaceand an inner peripheral surface formed by said thermosettingpolyurethane layer, wherein a polyurethane layer forming said outerperipheral surface is made of a composition containing a urethaneprepolymer having isocyanate groups on ends and a hardener containingdimethylthiotoluenediamine.
 2. The papermaking belt according to claim1, wherein said urethane prepolymer and said hardener are mixed witheach other in a ratio setting the value of the equivalent ratio (H/NCO)between active hydrogen of said hardener and the isocyanate groups (NCO)of said urethane prepolymer to 1<H/NCO<1.15.
 3. A papermaking beltcomprising a reinforcing substrate embedded in a thermosettingpolyurethane layer and having an outer peripheral surface and an innerperipheral surface formed by said thermosetting polyurethane layer,wherein a polyurethane layer forming said outer peripheral surface ismade of a composition containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on ends, andsaid urethane prepolymer and said hardener are mixed with each other ina ratio setting the value of the equivalent ratio (H/NCO) between theactive hydrogen groups (H) of said hardener and the isocyanate groups(NCO) of said urethane prepolymer to 1<H/NCO<1.15.
 4. A papermaking beltcomprising a reinforcing substrate embedded in a thermosettingpolyurethane layer, said thermosetting polyurethane layer including aninner polyurethane layer and an outer polyurethane layer adhering to theouter peripheral surface of said inner polyurethane layer, wherein eachof said inner polyurethane layer and said outer polyurethane layer ismade of a composition containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on ends, thecomposition forming said inner polyurethane layer is prepared by mixingthe urethane prepolymer and the hardener with each other in a ratiosetting the equivalent ratio (H/NCO) between the active hydrogen groups(H) of the hardener and the isocyanate groups (NCO) of the urethaneprepolymer to 0.85≦H/NCO<1, and the composition forming said outerpolyurethane layer is prepared by mixing the urethane prepolymer and thehardener with each other in a ratio setting the value of said equivalentratio (H/NCO) to 1<H/NCO<1.15.
 5. The papermaking belt according toclaim 4, wherein the adhesive surface between said inner polyurethanelayer and said outer polyurethane layer is present in said reinforcingsubstrate, the urethane prepolymer forming said inner polyurethane layercontains a urethane prepolymer obtained by reacting polyol and diphenylmethane diisocyanate (MDI) with each other, and the urethane prepolymerforming said outer polyurethane layer contains a urethane prepolymerobtained by reacting polyol and tolylene diisocyanate (TDI) with eachother.
 6. The papermaking belt according to claim 5, wherein at least 50wt. % of the hardener forming said inner polyurethane layer is polyol.7. The papermaking belt according to claim 5 or 6, wherein saidreinforcing substrate contains multi-woven fabric.
 8. The papermakingbelt according to any of claims 4 to 6, wherein said outer polyurethanelayer adheres to the outer peripheral surface of said inner polyurethanelayer and forms the outer peripheral surface of the papermaking belt,and is made of a composition containing a urethane prepolymer havingisocyanate groups on ends and a hardener containingdimethylthiotoluenediamine.
 9. The papermaking belt according to any ofclaims 4 to 6, wherein said thermosetting polyurethane layer includes aninner polyurethane layer, an outer polyurethane layer adhering to theouter peripheral surface of said inner polyurethane layer and apolyurethane layer located on the outer side of said outer polyurethanelayer for forming the outer peripheral surface of the papermaking belt,and said polyurethane layer forming the outer peripheral surface is madeof a composition containing a urethane prepolymer having isocyanategroups on ends and a hardener containing dimethylthiotoluenediamine. 10.The papermaking belt according to claim 9, wherein the composition ofsaid polyurethane layer forming the outer peripheral surface is preparedby mixing said urethane prepolymer and said hardener with each other ina ratio setting the value of the equivalent ratio (H/NCO) between theactive hydrogen groups (H) of said hardener and the isocyanate groups(NCO) of said urethane prepolymer to 1<H/NCO<1.15.
 11. The papermakingbelt according to any of claims 1 to 6, wherein said polyurethane layeris hardened at a temperature of 120° C. to 140° C.
 12. The papermakingbelt according to any of claims 1 to 6, provided with grooves on theouter peripheral surface.
 13. A method of manufacturing a papermakingbelt including an inner polyurethane layer and an outer polyurethanelayer adhering to the outer peripheral surface of said innerpolyurethane layer by embedding a reinforcing substrate in athermosetting polyurethane layer thereby integrating said reinforcingsubstrate and said thermosetting polyurethane layer with each other,including: a first step of hardening a liquid mixture, containing aurethane prepolymer having isocyanate groups on ends and a hardenerhaving active hydrogen groups on ends, prepared by mixing the urethaneprepolymer and the hardener with each other in a ratio setting the valueof the equivalent ratio (H/NCO) between the active hydrogen groups (H)of the hardener and the isocyanate groups (NCO) of the urethaneprepolymer to 0.85≦H/NCO<1 at a temperature of 70° C. to 100° C. forforming said inner polyurethane layer; a second step of applying aliquid mixture, containing a urethane prepolymer having isocyanategroups on ends and a hardener having active hydrogen groups on ends,prepared by mixing the urethane prepolymer and the hardener with eachother in a ratio setting the value of the equivalent ratio (H/NCO)between the active hydrogen groups (H) of the hardener and theisocyanate groups (NCO) of the urethane prepolymer to 1<H/NCO<1.15 ontothe outer peripheral surface of said inner polyurethane layer; and athird step of heating the whole to a temperature of 120° C. to 140° C.for hardening the liquid mixture applied onto the outer peripheralsurface of the inner polyurethane layer and forming the outerpolyurethane layer while bonding and integrating the inner polyurethanelayer and the outer polyurethane layer to and with each other.
 14. Themethod of manufacturing a papermaking belt according to claim 13,wherein said reinforcing substrate is impregnated with said innerpolyurethane layer from one surface side of said reinforcing substrateto an intermediate portion of the thickness of said reinforcingsubstrate, and said reinforcing substrate is impregnated with said outerpolyurethane layer from the other surface side of said reinforcingsubstrate to the position where said reinforcing substrate isimpregnated with said inner polyurethane layer.
 15. The method ofmanufacturing a papermaking belt according to claim 14, wherein saidreinforcing substrate contains multi-woven fabric.
 16. The method ofmanufacturing a papermaking belt according to claim 13, including a stepof winding said reinforcing substrate on the outer peripheral surface ofsaid inner polyurethane layer before or after hardening said innerpolyurethane layer.